What is an Accept-Encoding header?
Tells the server which content encodings (e.g. gzip, br) the client can decode.
Accept-Encoding is the request-side half of compression - the client lists which algorithms (gzip, Brotli, etc.) it can decompress, and a well-configured website responds with Content-Encoding set to whichever it actually used. If your uptime monitor doesn't send a realistic Accept-Encoding header, you risk measuring the uncompressed response time and getting numbers that don't match what real visitors experience.
Common use case
A CDN reads Accept-Encoding to decide whether to serve a visitor the Brotli-compressed, gzip-compressed, or uncompressed variant of a cached asset.
Example
Accept-Encoding: gzip, deflate, br, zstdHistory
Part of the original HTTP/1.0 specification, RFC 1945 (1996); Brotli and Zstandard support were added later as those algorithms matured.
Did you know?
A poorly configured cache that ignores this header when storing responses can serve a Brotli-compressed page to a client that only understands gzip - a real, if uncommon, source of "the page won't load" bug reports.